Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 261: Which is the first enzyme to be elevated in myocardial infarction?

A. CPK-MB
B. LDH
C. Myoglobin (Correct Answer)
D. Troponin-I

Explanation: **Explanation:** In the setting of Myocardial Infarction (MI), the timing of cardiac marker release depends on the size of the molecule and its location within the cardiac cell. **Why Myoglobin is the correct answer:** Myoglobin is a small heme protein found in the cytosol of cardiac and skeletal muscle. Due to its low molecular weight, it is released rapidly into the bloodstream following cell injury. It is the **earliest marker** to rise, appearing within **1–3 hours** of symptom onset, peaking at 6–9 hours, and returning to baseline within 24 hours. While highly sensitive for early detection, it lacks cardiac specificity as it also rises in skeletal muscle injury. **Analysis of Incorrect Options:** * **CPK-MB:** This isoenzyme begins to rise **4–6 hours** after infarction, peaks at 24 hours, and returns to normal in 48–72 hours. It is the gold standard for detecting **re-infarction**. * **Troponin-I:** These are the most **specific** markers for MI. They begin to rise at **3–6 hours**, peak at 12–24 hours, and remain elevated for 7–10 days. * **LDH (Lactate Dehydrogenase):** This is a late marker. It begins to rise after **12–24 hours**, peaks at 48–72 hours, and stays elevated for 10–14 days. **High-Yield Clinical Pearls for NEET-PG:** * **Earliest Marker:** Myoglobin. * **Most Specific Marker:** Cardiac Troponins (I and T). * **Marker for Re-infarction:** CPK-MB (due to its short half-life). * **Late Marker:** LDH (specifically the LDH-1 > LDH-2 "flipped pattern"). * **First enzyme to rise:** Though Myoglobin is the first *marker*, if the question specifically asks for the first **enzyme**, the answer is **CPK-MB** (Myoglobin is a protein, not an enzyme). However, in most competitive exams, Myoglobin is the expected answer for "first marker to rise."

Question 262: Which of the following is considered a cardiac enzyme?

A. Creatine phosphokinase (CPK) (Correct Answer)
B. Lactate dehydrogenase (LDH)
C. Serum glutamic-oxaloacetic transaminase (SGOT)
D. Alkaline Phosphatase

Explanation: **Explanation:** The correct answer is **Creatine phosphokinase (CPK)**, specifically the **CK-MB** isoenzyme. While several enzymes are released into the bloodstream following myocardial injury, CPK (specifically CK-MB) is historically the most specific "cardiac enzyme" among the options provided for diagnosing Myocardial Infarction (MI). **Why CPK is the correct answer:** Creatine phosphokinase exists in three isoforms: MM (skeletal muscle), BB (brain), and **MB (cardiac muscle)**. CK-MB rises within 4–6 hours of an MI, peaks at 24 hours, and returns to baseline within 48–72 hours. Its rapid clearance makes it the gold standard for detecting **re-infarction**. **Analysis of Incorrect Options:** * **Lactate dehydrogenase (LDH):** While LDH levels rise in MI (specifically LDH-1 > LDH-2, known as the "flipped pattern"), it is highly non-specific as it is found in RBCs, liver, and kidneys. It is now rarely used clinically for cardiac workups. * **SGOT (AST):** Serum glutamic-oxaloacetic transaminase was the first biomarker used for MI. However, it is abundant in the liver and skeletal muscle, making it non-specific for cardiac injury. * **Alkaline Phosphatase (ALP):** This is a marker for cholestatic liver disease and bone turnover; it has no diagnostic role in cardiac pathology. **NEET-PG High-Yield Pearls:** * **Troponin I/T:** Currently the **most sensitive and specific** markers for MI (not listed in the options). * **Myoglobin:** The **earliest** marker to rise (within 1–2 hours) but lacks specificity. * **CK-MB:** Best marker for **re-infarction** due to its short half-life. * **LDH-1:** The "flipped ratio" (LDH-1 > LDH-2) is characteristic of myocardial damage or hemolysis.

Question 263: H2O2 is broken down or formed by which of the following enzymes?

A. Oxidase (Correct Answer)
B. Oxygenase
C. Hydrolase
D. None of the above

Explanation: **Explanation:** Enzymes involved in oxidation-reduction reactions are classified as **Oxidoreductases**. The question focuses on the specific handling of Hydrogen Peroxide ($H_2O_2$). **1. Why Oxidase is Correct:** Oxidases catalyze the removal of hydrogen from a substrate using **oxygen as a hydrogen acceptor**. In many of these reactions, the oxygen is reduced specifically to **Hydrogen Peroxide ($H_2O_2$)** rather than water. A classic example is *Xanthine Oxidase*. Additionally, the enzyme **Peroxidase** (a sub-type of oxidase) specifically breaks down $H_2O_2$ into water. Therefore, oxidases are the primary group associated with the formation or degradation of $H_2O_2$. **2. Why Other Options are Incorrect:** * **Oxygenases:** These enzymes do not produce $H_2O_2$. Instead, they catalyze the **incorporation of oxygen atoms** directly into the substrate molecule. They are divided into Dioxygenases (incorporate both atoms of $O_2$) and Monooxygenases (incorporate one atom as -OH and reduce the other to $H_2O$). * **Hydrolases:** These enzymes catalyze the cleavage of bonds (C-O, C-N, C-C) by the **addition of water**. They are not involved in redox reactions or $H_2O_2$ metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Catalase:** A hemoprotein containing four heme groups. it is one of the fastest enzymes known and specifically protects the body from oxidative damage by breaking down $H_2O_2$ into $H_2O$ and $O_2$. * **Superoxide Dismutase (SOD):** Converts the superoxide free radical ($O_2^-$) into $H_2O_2$ and $O_2$. * **Glutathione Peroxidase:** A **Selenium-dependent** enzyme that helps neutralize $H_2O_2$ in RBCs, protecting them from hemolysis. * **Peroxisomes:** The cellular organelles where most $H_2O_2$-producing oxidases are localized.

Question 264: The debranching enzyme in glycogenolysis hydrolyzes which one of the following bonds to release free glucose?

A. alpha (1-4) glycosidic bond
B. alpha (1-6) glycosidic bond (Correct Answer)
C. beta (1-4) glycosidic bond
D. 3 (1-6) glycosidic bond

Explanation: ### Explanation The **Debranching Enzyme** is a bifunctional protein essential for the complete breakdown of glycogen. It possesses two distinct catalytic activities that act once **Glycogen Phosphorylase** has shortened the glucose chains to four residues from a branch point (a structure known as *limit dextrin*). 1. **4-alpha-D-glucanotransferase activity:** It moves a trisaccharide unit from one outer branch to the end of another, exposing the single glucose residue attached by an **alpha (1-6) bond**. 2. **Amylo-alpha (1-6)-glucosidase activity:** This specific component of the enzyme hydrolyzes the remaining alpha (1-6) glycosidic bond. Unlike phosphorylase (which produces glucose-1-phosphate), this hydrolytic step releases **free glucose**. This accounts for approximately 8–10% of the glucose released from glycogen. #### Analysis of Incorrect Options: * **Option A: alpha (1-4) glycosidic bond:** These are the linear bonds in glycogen. They are cleaved by **Glycogen Phosphorylase** via phosphorolysis (not hydrolysis) to produce Glucose-1-Phosphate. * **Option C: beta (1-4) glycosidic bond:** These bonds are found in **cellulose**, not glycogen. Humans lack the cellulase enzyme to digest these. * **Option D: 3 (1-6) glycosidic bond:** This is a distractor; the branching in glycogen specifically involves the 1st and 6th carbon atoms in an alpha configuration. #### High-Yield Clinical Pearls for NEET-PG: * **Cori’s Disease (GSD Type III):** Caused by a deficiency of the Debranching Enzyme. It presents with hepatomegaly, growth retardation, and fasting hypoglycemia. Unlike von Gierke’s, blood lactate levels are usually normal. * **Product Ratio:** For every 10–12 molecules of Glucose-1-Phosphate produced by phosphorylase, 1 molecule of free glucose is produced by the debranching enzyme. * **Location:** Glycogenolysis occurs in the **cytosol**.

Question 265: Which organ is known to show elevated levels of Lactate Dehydrogenase 5 (LDH5)?

A. Kidney
B. Heart
C. Liver (Correct Answer)
D. Prostate

Explanation: **Explanation:** Lactate Dehydrogenase (LDH) is a tetrameric enzyme composed of two types of subunits: **H (Heart)** and **M (Muscle)**. These combine to form five distinct isoenzymes (LDH1 to LDH5), which exhibit tissue-specific distribution based on their subunit composition. **1. Why Liver is Correct:** **LDH5 (M4)** consists of four M subunits. It is primarily found in the **liver** and **skeletal muscle**. Because LDH5 is the "slowest" moving isoenzyme on electrophoresis, its elevation in serum is a specific marker for hepatocellular injury (e.g., hepatitis, cirrhosis) or skeletal muscle diseases (e.g., muscular dystrophy). **2. Analysis of Incorrect Options:** * **Heart (LDH1 - H4):** The heart predominantly contains LDH1. In myocardial infarction, LDH1 levels rise, often leading to the "flipped pattern" where LDH1 becomes higher than LDH2. * **Kidney (LDH1 & LDH2):** The renal cortex is rich in LDH1 and LDH2. While the kidney contains some LDH5 in the medulla, it is not the characteristic or predominant site. * **Prostate:** While the prostate contains LDH, it is not a primary diagnostic site for LDH5. The clinical marker of choice for the prostate is PSA (Prostate-Specific Antigen) or Acid Phosphatase. **3. High-Yield Clinical Pearls for NEET-PG:** * **Electrophoretic Mobility:** LDH1 moves fastest toward the anode (+), while **LDH5 moves slowest**. * **LDH2 (H3M1):** This is the most abundant isoenzyme in normal human serum. * **LDH4 (HM3):** Primarily found in the lungs, placenta, and pancreas. * **LDH-X (LDH6):** A sixth isoenzyme found in mature spermatozoa/testes. * **Diagnostic Significance:** LDH is a non-specific marker of cell death; however, isoenzyme fractionation is crucial for localizing the site of pathology.

Question 266: The enzyme carbamoyl phosphate synthetase requires:

A. Mg++ (Correct Answer)
B. Ca++
C. K+
D. None of the above

Explanation: **Explanation:** **Carbamoyl Phosphate Synthetase (CPS)** is a critical enzyme in nitrogen metabolism. There are two isoforms: **CPS-I** (found in mitochondria for the Urea Cycle) and **CPS-II** (found in the cytosol for Pyrimidine Synthesis). Both isoforms catalyze reactions that are highly energy-dependent, requiring the hydrolysis of **ATP**. **Why Mg++ is the correct answer:** Most enzymes that utilize ATP as a substrate require **Magnesium (Mg++)** as a mandatory cofactor. Mg++ binds to the phosphate groups of ATP, neutralizing their negative charge and facilitating the nucleophilic attack. Specifically, for CPS-I, Mg++ is essential for the activation of bicarbonate and the subsequent phosphorylation steps. Without Mg++, the enzyme cannot stabilize the high-energy intermediates required to form carbamoyl phosphate. **Why other options are incorrect:** * **Ca++ (Calcium):** While calcium is a vital secondary messenger and a cofactor for enzymes like α-amylase and certain clotting factors, it does not typically act as a cofactor for ATP-dependent ligases like CPS. * **K+ (Potassium):** Potassium is the major intracellular cation and is required for the activity of specific enzymes like **Pyruvate Kinase**, but it does not play a direct role in the CPS reaction mechanism. **High-Yield Clinical Pearls for NEET-PG:** * **CPS-I vs. CPS-II:** Remember that CPS-I requires **N-Acetylglutamate (NAG)** as an absolute allosteric activator. CPS-II does not require NAG. * **Location:** CPS-I is in the **Mitochondria** (Mnemonic: **M**itochondria for **M**etabolism of Ammonia/Urea); CPS-II is in the **Cytosol** (Mnemonic: **C**ytosol for **C**yrimidine synthesis). * **Rate-Limiting Step:** CPS-I is the rate-limiting enzyme of the Urea Cycle. Deficiency leads to **Type I Hyperammonemia**, characterized by neurological symptoms and low citrulline levels.

Question 267: Which of the following enzymes is FAD-linked?

A. Isocitrate dehydrogenase
B. Pyruvate dehydrogenase
C. Succinate dehydrogenase (Correct Answer)
D. Enoyl reductase

Explanation: **Explanation:** **Succinate Dehydrogenase (SDH)** is the correct answer because it is a unique enzyme of the TCA cycle that is directly embedded in the inner mitochondrial membrane (acting as **Complex II** of the Electron Transport Chain). Unlike most other dehydrogenases that use NAD+, SDH requires **FAD (Flavin Adenine Dinucleotide)** as its prosthetic group. This is because the free energy change associated with the oxidation of succinate to fumarate is insufficient to reduce NAD+ but is adequate to reduce FAD to FADH₂. **Analysis of Incorrect Options:** * **Isocitrate Dehydrogenase:** This is the rate-limiting enzyme of the TCA cycle and primarily uses **NAD+** (mitochondrial) or NADP+ (cytosolic) as a coenzyme. * **Pyruvate Dehydrogenase (PDH):** While the PDH complex *contains* FAD (as part of the E3 subunit), it is a multi-enzyme complex where the final electron acceptor that leaves the complex is **NAD+** (forming NADH). In the context of "FAD-linked" in competitive exams, SDH is the classic textbook example of a direct FAD-dependent reaction. * **Enoyl Reductase:** This enzyme is part of the Fatty Acid Synthase complex and utilizes **NADPH** as a reducing agent for fatty acid synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Dual Role:** SDH is the only enzyme that participates in both the **TCA Cycle** and the **Electron Transport Chain**. * **Inhibitor:** **Malonate** is a classic competitive inhibitor of Succinate Dehydrogenase due to its structural similarity to succinate. * **Riboflavin Link:** Since FAD is derived from **Vitamin B2 (Riboflavin)**, deficiencies in this vitamin directly impair SDH activity. * **Marker Enzyme:** SDH is often used as a marker enzyme for the inner mitochondrial membrane.

Question 268: Which enzyme splits hydrogen peroxide into water and oxygen?

A. Cytochromes
B. Cytochrome P450
C. Superoxide dismutase
D. Catalase (Correct Answer)

Explanation: **Explanation:** **1. Why Catalase is the correct answer:** Catalase is a hemeprotein found primarily in **peroxisomes**. Its primary function is to protect cells from oxidative damage by catalyzing the decomposition of hydrogen peroxide ($H_2O_2$) into water ($H_2O$) and molecular oxygen ($O_2$). The reaction is: $2H_2O_2 \rightarrow 2H_2O + O_2$. It has one of the highest turnover numbers among all enzymes, processing millions of molecules per second. **2. Analysis of Incorrect Options:** * **Cytochromes (A):** These are heme-containing proteins involved in the Electron Transport Chain (ETC). They function as electron carriers (undergoing $Fe^{2+}/Fe^{3+}$ redox cycles) rather than decomposing peroxides. * **Cytochrome P450 (B):** This superfamily of enzymes is primarily involved in the hydroxylation of drugs and steroids (Phase I metabolism) in the endoplasmic reticulum of the liver. * **Superoxide dismutase (C):** This enzyme acts on the **superoxide radical** ($O_2^{\bullet-}$), converting it into hydrogen peroxide ($H_2O_2$) and oxygen. It precedes the action of catalase in the antioxidant defense pathway. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Glutathione Peroxidase:** Another key enzyme that neutralizes $H_2O_2$ in the cytosol; it requires **Selenium** as a cofactor. * **Catalase Test:** Used in Microbiology to differentiate *Staphylococci* (Catalase positive) from *Streptococci* (Catalase negative). * **Acatalasia:** A rare genetic deficiency of catalase leading to oral ulcerations and gangrene. * **Peroxisomes:** Known as "microbodies," they contain catalase and are essential for the $\beta$-oxidation of Very Long Chain Fatty Acids (VLCFA).

Question 269: Which of the following enzymes is active in its phosphorylated form?

A. Pyruvate dehydrogenase
B. HMG-CoA reductase
C. HMG-CoA reductase kinase (Correct Answer)
D. Pyruvate kinase

Explanation: **Explanation:** The metabolic regulation of enzymes via covalent modification (phosphorylation/dephosphorylation) is a high-yield concept. In general, **catabolic enzymes** (breakdown) are active when phosphorylated, while **anabolic enzymes** (synthesis) are active when dephosphorylated. **Why Option C is Correct:** **HMG-CoA reductase kinase** (also known as AMP-activated protein kinase or AMPK) is part of a phosphorylation cascade. Its role is to inhibit cholesterol synthesis when cellular energy is low. To perform its function of phosphorylating (and thus inactivating) HMG-CoA reductase, the kinase itself must be **activated by phosphorylation**. This is an exception to the general rule that "kinases" are the actors; here, the regulator itself is regulated. **Analysis of Incorrect Options:** * **A. Pyruvate Dehydrogenase (PDH):** This enzyme links glycolysis to the TCA cycle. It is **inactivated** by phosphorylation (via PDH kinase) and activated by dephosphorylation (via PDH phosphatase). * **B. HMG-CoA Reductase:** This is the rate-limiting enzyme for cholesterol synthesis. It follows the "anabolic rule": it is **active in the dephosphorylated state** and inactive when phosphorylated. * **D. Pyruvate Kinase:** A key glycolytic enzyme. In the liver, it is **inactivated** by phosphorylation (stimulated by glucagon) to prevent glycolysis during fasting. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "P" for Phosphorylated = "P" for Pumped up (Catabolic) / "D" for Dephosphorylated = "D" for Done building (Anabolic). * **Key Exception:** Glycogen Phosphorylase is **active** when phosphorylated (catabolic), whereas Glycogen Synthase is **inactive** when phosphorylated (anabolic). * **HMG-CoA Reductase** is the target of **Statins**, which are competitive inhibitors used in dyslipidemia.

Question 270: Which of the following inhibitors in the TCA cycle acts by blocking citrate?

A. Fluoroacetate (Correct Answer)
B. Arsenite
C. Malonate
D. None of the above

Explanation: ### Explanation **1. Why Fluoroacetate is Correct:** Fluoroacetate is a classic example of **suicide inhibition** (mechanism-based inhibition). In the TCA cycle, fluoroacetate is first converted to **fluoroacetyl-CoA**, which then reacts with oxaloacetate via the enzyme citrate synthase to form **fluorocitrate**. Fluorocitrate is a potent inhibitor of the enzyme **Aconitase**. By inhibiting aconitase, it prevents the conversion of citrate to isocitrate, leading to a toxic accumulation of **citrate** in the mitochondria and halting the cycle. **2. Why Other Options are Incorrect:** * **Arsenite (B):** Arsenite inhibits enzymes that require **lipoic acid** as a cofactor. In the TCA cycle, its primary target is the **$\alpha$-ketoglutarate dehydrogenase** complex. It does not block citrate; rather, it leads to the accumulation of $\alpha$-ketoglutarate. * **Malonate (C):** Malonate is a classic **competitive inhibitor** of **Succinate Dehydrogenase** (Complex II). It has a structural similarity to succinate and competes for the enzyme's active site, blocking the conversion of succinate to fumarate. **3. High-Yield Clinical Pearls for NEET-PG:** * **Suicide Inhibition:** Remember that fluoroacetate itself is not toxic; it becomes toxic only after being metabolized by the target cell (lethal synthesis). * **Arsenic Poisoning:** Arsenite also inhibits the **Pyruvate Dehydrogenase (PDH)** complex, leading to lactic acidosis and a "garlic breath" odor. * **Aconitase:** This enzyme contains an **iron-sulfur (Fe-S) cluster**, making it sensitive to oxidative stress and inhibition by fluorocitrate. * **Summary of TCA Inhibitors:** * Fluoroacetate $\rightarrow$ Aconitase * Arsenite $\rightarrow$ $\alpha$-Ketoglutarate Dehydrogenase * Malonate $\rightarrow$ Succinate Dehydrogenase

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Enzymes — MCQs

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